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Adding weight to the rear of the vehicle would not make it grip the ground any better than reducing the weight in terms of skidding outward. If you increase the weight, the outward pull would increase as well, so overall, the effect would remain constant.
When you add weight, it adds a force downward, not inward, so it would not give it better grip. But doing so, out of coincidence probably pushes your tires down so they become wider, increasing the surface area, thus more grip.
Here it is in the form of equations, again just some simple physics if you've taken a highschool course of Physics.
(force caused by friction) = (coefficient of friction)(normal force)
F = uN
N = mg so F = umg
a = acceleration toward the center
but then F = ma so
(applied force toward the center) = m(normal acceleration)
ma=umg <-- since m's cancel, increasing mass does not increase friction
a = ug
acceleration normal toward the center
you just gotta make the material more rough by increasing its coefficient of friction. Since rubber has the highest(and is the most economical material) coefficient of friction on paved roads.
In regards to oversteering and understeering, adding weight in the back would counter the added engine weight in the front(which would cause understeer) by adding oversteer. When you add weight, it also depends on where you add it. If you put it in the trunk(vs on top of the trunk), I think it's safe to assume that it is below the center of gravity, (taken from all 3 dimensions i, j, and k directions). If that is so, it would reduce roll for you as well.
As for what you can do to reduce this problem(well not really a problem. only 85 lbs huh?), just test and see what benefits the most. When you brake, the weight transfers to the front and the back lifts up(reducing grip from the rear tires). How would you tune it so that the rear goes up the least possible? I wouldn't loosen the rear because don't the coilovers pull the car down and prevent roll? Loosening it would allow the back to come up more.
I think the only noticeable difference would be braking distance. Distance will be a little longer caused by loss of grip in the rear tires. Unless you're drifting your car I don't think you'll notice that much difference.
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